It is an erect or spreading shrub with densely hairy branches that are more or less square in cross-section, narrow egg-shaped or oblong leaves with the edges rolled under, and deep mauve to purple flowers with maroon dots inside the petal tube.

Prostanthera galbraithiae is an erect or spreading shrub that typically grows to a height of 0.1–2 m (3.9 in – 6 ft 6.7 in) and has densely hairy branches that are more or less square in cross-section.

[5] The specific epithet is named for Jean Galbraith, a member of the Latrobe Valley Field Naturalists, who co-discovered the species and advocated for its protection.

It is associated with Eucalyptus obliqua woodland with a heathy understorey including species such as Acacia oxycedrus, Epacris impressa, Lepidosperma concavum, Leptospermum myrsinoides and Platylobium obtusangulum.

[2][6] The main threats to the species include inappropriate fire regimes, firebreak maintenance, herbicide use and competition from bracken fern Pteridium esculentum.